A conventional oil well includes a cased wellbore with at least one string of tubing extending downwardly through the casing into the oil or other petroleum fluid contained in the subsurface mineral formation to be produced. The casing is perforated at the level of the production zone to permit fluid flow from the formation into the casing, and the lower end of the tubing string is generally open to provide entry for the fluid in the tubing.
Many hydrocarbon wells are unable to produce at commercially viable levels without assistance in lifting the formation fluids to the earth's surface. In some instances, high fluid viscosity inhibits fluid flow to the surface. More commonly, formation pressure is inadequate to drive fluids upward in the wellbore. In the case of deeper wells, extraordinary hydrostatic head acts downwardly against the formation and inhibits the unassisted flow of production fluid to the surface.
In many instances, artificial lift may be required to raise the produced fluids to the surface. A common approach for urging production fluids to the surface uses a mechanically actuated, positive displacement pump driven from the surface by a pumpjack connected to the pump by a sucker rod string. Reciprocal movement of the sucker rod string induces reciprocal movement of the pump for lifting production fluid to the surface.
For example, a reciprocating rod lift system 20 of the prior art is shown in FIG. 1A to produce production fluid from a wellbore 10. As is typical, surface casing 12 hangs from the surface and has a liner casing 14 hung therefrom by a liner hanger 16. Production fluid F from the formation 11 outside the cement 13 can enter the liner 14 through perforations 17. To convey the fluid, production tubing 26 extends from a wellhead 28 downhole, and a packer 15 seals the annulus between the production tubing 26 and the liner 14. At the surface, the wellhead 28 receives production fluid and diverts it to a flow line 29.
The production fluid F may not produce naturally to reach the surface so operators use the reciprocating rod lift system 20 to lift the fluid F. The system 20 has a surface pumping unit 22, a sucker rod string 24, and a downhole rod pump 30. The surface pumping unit 22 reciprocates the rod string 24, and the reciprocating string 24 operates the downhole rod pump 30. The rod pump 30 has internal components attached to the rod string 24 and has external components positioned in a pump-seating nipple 31 near the producing zone and the perforations 17.
As best shown in the detail of FIG. 1B, the rod pump 30 has a barrel 40 with a plunger 32 movably disposed therein. The plunger 32 has a plunger rod 33 attached to it, which connects to the rod string (24; FIG. 1A) of the reciprocating rod lift system 20. The plunger rod 33 is of sufficient length so that the plunger rod 33 will extend through the upper end of the barrel 40 even at the bottom of the plunger's stroke.
The barrel 40 has a standing valve 42, and the plunger 32 has a traveling valve 34. For example, the standing valve 42 disposed in the barrel 40 can be a check valve having a ball 44 and seat 46. Similarly, the traveling valve 34 can also be a check valve (i.e., one-way valve) having a ball 36 and seat 38. For its part,
As the surface pumping unit 22 in FIG. 1A reciprocates, the rod string 24 reciprocates in the production tubing 26 and moves the plunger 32. The plunger 32 moves the traveling valve 34 in reciprocating upstrokes and downstroke. During an upstroke, the traveling valve 34 as shown in FIG. 1B is closed (i.e., the upper ball 36 seats on upper seat 38). In many instances, the force acting on the plunger 32 through the sucker rod string 24 may exceed 100,000 pounds.
Movement of the closed traveling valve 34 upward reduces the static pressure within the pump chamber 45b (the volume between the standing valve 42 and the traveling valve 32 that serves as a path of fluid transfer during the pumping operation). This, in turn, causes the standing valve 42 to unseat so that the lower ball 44 lifts off the lower seat 46. Production fluid F is then drawn upward into the chamber 45b. 
Ultimately, the produced fluid F is delivered by positive displacement of the plunger 32, out passages 45a in the barrel 40. The moved fluid then moves up the wellbore 10 through the tubing 26 as shown in FIG. 1A.
On the following downstroke, the plunger 32 moves downward in barrel 40 by the reciprocation applied by the pumping unit 22 via the sucker rod string 24. The weight of the sucker rod string 24 pushes the plunger 32 through the fluid in the barrel 40. The standing valve 42 closes as the standing ball 44 seats upon the lower seat 46. At the same time, the traveling valve 34 opens so fluids previously residing in the chamber 45b can pass through the valve 34 and into the plunger 32. The upstroke and down stroke cycles are repeated, causing fluids to be lifted upward through the wellbore 10 and ultimately to the earth's surface.
At some point, it may become necessary to disconnect or connect the sucker rod string 24 with the pump 30, such as in an oversize tubing pump installation where the pump plunger 32 is installed separately from the sucker rod string 24. In an insert pump or a standard tubing pump 30, the plunger 32 or other portions of the pump 30 may become sanded in, corroded, or otherwise difficult to remove from the wellbore 10. Typically, the sucker rod string 24 is not robust enough to transmit the necessary force required to remove stuck components without damaging the sucker rod string 24 for later use. In other instances, it may be desirable the remove only the sucker rod string 24 simply to adjust and maintain the sucker rod string 24 without removing either the plunger 32 or the entire barrel pump 40.
For these reasons, it may be desirable to use a disconnect device or on-off tool 50 on the sucker rod string 24, as shown in FIGS. 1A-1B. The on-off tool 50 must be able to disconnect the sucker rod string 24 at the desired location, but must also be able to be reconnected as desired by the operators. Usually, the on-off tool 50 is installed on the sucker rod string 24 close to the plunger 32.
To connect the sucker rod string 24 to the pump 30 disposed downhole, the on-off tool 50 latches automatically to the pump 30 as the sucker rod string 24 is lowered. The on-off tool 50 then rotates to the correct alignment position and uses the sucker rod string's weight to complete the latching. To disconnect the sucker rod string 24 from the pump 30, the on-off tool 50 releases or unlatches by simply setting the pump 30 at the bottom of its stroke and turning the rod string 24 in the release direction of the tool 50 while slowly picking up the rod string 24. The on-off tool 50 can have either right-release or left-release direction based on the application and other equipment used.
A number of on-off tools 50 are available to connect/disconnect the sucker rod string 24 to the pump 30. For example, FIGS. 2 and 3 depict prior art sucker rod disconnects or on-off tools 50 for use on a reciprocating sucker rod string.
The on-off tool 50 of FIG. 2 is an example of a conventional on-off tool similar to that disclosed in U.S. Pat. No. 3,366,408. The tool 50 includes a housing 52 having a top fitting 54 with a pin connector for attaching to a sucker rod (24) with a coupling (25). Contained inside the housing 52, a pawl 58 can move axially/longitudinally against the bias of one or more springs 56. Spherical bearings 59a on the pawl 58 keep it from rotating as the bearings 59a ride in channels inside the housing 52. Opposite the housing 52, the tool 50 includes a key 70 having a head 72 at its distal end and connecting at its proximal end 74 with a box connector to the plunger rod (33) of a pump (30).
Connection is made when the housing 52 is lowered onto the key 70 so that the housing 52 interfits and interlocks with the key 70. The connection is accomplished by the weight of the sucker rod string (24) above the housing 52, by a rotation of the rod string (24) that causes relative rotary motion between the housing 52 and the key 70, and by the latching action of the key's head 72 to the internal mechanism of the tool 50.
In particular, the key 70 inserts in a key slot 60 in the housing 52 so the key 70 in a locked position can engage a seat or ledge 62 and transmit the tensile forces exerted by the pumpjack (22) on the up stroke. A spiral profile 64 in the housing 52 can help orient the insertion of the key's head 72 through the slot 60. The key's proximal end 74 cooperates with the distal end of the housing 52 to transmit any compressive forces of the tool 50 to the plunger rod (33) and ultimately the plunger (32).
The key 70 is typically inserted into the key slot 60 where the key 70 acts upon the pawl 58 to compress the spring 56. The key 70 is then rotated, typically about 90 degrees, allowing the spring 56 to extend and the pawl 58 to lower onto the key head (72), which places the key 70 into a locked position. The pawl 58 and the spring 56 then act upon the key 70 to prevent the key 70 from returning to the unlocked position until the operator desires to disconnect the sucker rod string (24) at the location of the tool 50. In use then, the key 70, pawl 58, and other components of the tool 50 allow the operator to disconnect the sucker rod string (24) or to reconnect the sucker rod string (24) to the pump (30), as desired.
Another on-off tool 50 according to the prior art shown in the cross-sectional view of FIG. 3 also includes a housing 52 with a top fitting 54 for attaching to a sucker rod string (24) using a coupling (25). A pawl 58 in the housing 52 can move longitudinally against the bias of one or more springs 56a-b. Rather than using spherical bearings, fixed pins 59b on the housing 52 can ride in slots in the pawl 58 to keep the pawl 58 from rotating inside the housing 52. Opposite the housing 52, the tool 50 similarly includes a key 70 having a head 72 at its distal end and connecting at its proximal end 74 to the plunger rod (33) of a pump (30).
Operation of this on-off tool 50 is similar to that discussed previously. In particular, FIGS. 4A-4D show the on-off tool 50 of FIG. 3 during stages of coupling. Initially, the key 70 is coupled to the plunger rod 33 of the pump (30) disposed downhole in the production tubing (14). The housing 52 is connected to the sucker rod string (24) using the coupling 25 and is lowered down the tubing string to the pump (30). Eventually, the housing 52 inserts over the key 70, which passes through the slot 60 in the housing 52.
The head 72 of the key 70 has an oblong cross-section. If the key 70 is not properly aligned with the opening for the slot 60, then relative rotation between the housing 52 and key 70 can align the head 72 with the slot 60. Passing up through the housing 52, the key 70 pushes the pawl 58 against the bias of the spring 56a-b, as shown in FIG. 4B.
The on-off tool 50 incorporates a cam-type system using the internal pawl 58 under the spring's force and being actuated (in a longitudinal direction) by rotating relative to the key 70. This imparts torque, which requires the guide pins 59b to counteract the torque and to keep the pawl 58 from rotating in the housing 52.
To complete the latching, the sucker rod (24) is rotated to rotate the housing 52. As shown, the bottom of the housing 52 can have a clutch shoulder to engage a tab or the like on the key 70 to indicate sufficient rotation. The pawl 58 turns with the housing 52 until a pocket in the pawl 58 aligns with the oblong head 72, and the springs 56a-b then push the pawl 58 over the head 72. Reciprocating of the sucker rod (24) can now operate the pump (30) while the on-off tool 50 holds the sucker rod string (24) to the plunger rod (33).
Primarily, current on-off tools as disclosed above have a small radius and contact surfaces underneath the key's head 72 for engaging the seat 62 of the housing 52. For example, FIGS. 5A-1 and 5A-2 illustrate perspective views of the key head 72 according to the prior art, FIG. 5B shows the key's head 72 of the prior art seated in the housing's seat 62, and FIG. 5C illustrates a diagram of the load bearing areas of the prior art key head 72.
FIG. 5D illustrates the geometry of the key's head 72 of the prior art. The head's outline 79 is relatively oval with long sides L and short sides W. Each edge of the short sides W is fully defined by a full radius r. This shape corresponds roughly to the shape of the key slot 60 of the housing 52, which can be readily formed by milling out material in the housing 52 with a drill bit having the proper radius r. By contrast, manufacture of the key's head 72 with this outline 79 is less straightforward and can require more careful machining.
The key 70 has a cylindrical stem 71 between the larger head 72 and proximal end 74. As noted previously, the head 72 has an oblong shape so it can insert into the slot 60. Therefore, the head 72 has thinned sides 73 where a bearing surface 75 of the head 72 is absent. As noted above, the key's head 72 inserts through the housing's slot 60, and relative rotation of about 90-degrees places the head's bearing surface 75 against the seat or ledge 62 inside the housing 52. Additionally, the side's of the pawl (58) fit over the head 72 on its thinned sides 73.
As can be seen, current on-off tools 50 as disclosed above utilize coil springs 56 to bias the pawl 58 toward the key head 72 to complete the latching sequence. These coil springs 56 fail to provide enough axial load needed to keep the tool 100 latched in higher speed pumping applications where the dynamics of the sucker rod string 24 can cause the key 70 to overcome the rotational torque needed to compress the coil spring 56, causing the tool 100 to unlatch.
Unfortunately, it is apparent that the on-off tools 50 currently used in the industry can be the weakest part of the sucker rod string 24, thereby becoming the point in the sucker rod string 24 most likely to fail. The currently used tool 50 becomes weaker over time due to the loading and unloading of the tool 50, which can experience loads in excess of 100,000 pounds several times each minute for months or even years.
In particular, as with all components in the reciprocating system, the on-off tool 50 is subject to axial fatigue, which limits its lifespan in certain operating conditions. Currently, such tools use point-loading, which is undesirable.
During use, for example, the key 70 is subjected to deformation due to downstroke and upstroke impacts. The housing's seat 62 deforms due to broaching of the key head 72, and the seating area 75 of the key head 72 deforms from impact wear on upstrokes. Likewise, the housing 52 is subjected to brinelling due to impact on upstrokes.
As wear increases, the gap or play between the housing's bottom shoulder 57 and the key's ledge 77 increases and produces a slide hammer effect. The increased play between the housing 52 and the key 70 further beats the seat 62 against the head's bearing surface 75. Eventually, the key head 72 can break off due to impacts. During high stroke speeds, the current on-off tool 50 can also become unlatched due to dynamic forces (axial loads and torque) imparted through the rod string 24 coupled with low spring force on the pawl 58.
Carpenter tool offers a cam-type on/off tool that incorporates a large radius underneath the key head to reduce the stress concentration. This helps somewhat, but fails to address the high bearing contact stresses underneath the head. Thus, a need exists for a sucker rod disconnect that allows the sucker rod string to be disconnected at a particular point but that may then be reconnected without the disconnect becoming the weakest point in the sucker rod assembly, and thereby becoming the most likely failure point in the sucker rod string. Additionally, what is needed is an on-off tool that does not unlatch unexpectedly during normal use.
The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.